It is common to provide insulation for buildings, especially residences, and transport vehicles (land vehicles, maritime vehicles and aircraft), and for industrial structures such as ducts, utilizing cellular synthetic resin insulating compositions which generally are applied in the form of a liquid foam which reacts and hardens to produce the cellular product.
The reaction can be carried out in a factory by treating the industrial structure to be insulated or at the site in the case of building structures, either by casting the reactive liquid composition in a space to be provided with the insulating material, by injection of the reactive liquid into a cavity to be filled thereby, or by projecting the composition against a surface to be coated. Generally speaking these liquid reactive compositions have been urea-formaldehyde resins, polyurethane resins, epoxy resins or phenol-aldehyde (phenol-formaldehyde) resins or combinations thereof.
Because of the simplicity of application, urea-formaldehyde resins have been widely used for over 20 years in Europe and North America for the insulation of private domiciles and other structures.
Generally the reactive composition is discharged from a machine which proportions the two components in a fixed ratio with respect to one another, e.g. a 50/50 proportion by volume. The supply reservoirs of the machine thus can contain the two components, one of which is the resin in the form of an aqueous solution of a precondensate of urea-formaldehyde containing a plasticizer. The other composition can be an aqueous solution of the catalyst and the foaming agent which can include an anionic surface active agent in an acid medium. The two components are mixed, foamed in the presence of air and the resulting composition, in the form of a cream having the consistency of shaving cream, can be cast between slabs, can be injected through small holes in the space between partitions and walls, or can be coated onto structures to be provided with insulating layers.
The foam hardens in about one minute to leave a cellular material.
Initially the density of the foam depending upon the humidity, is about 40 to 48 kg/m.sup.3 but after drying over a period of several days to several weeks, the density diminishes to a level between 10 and 13 kg/m.sup.3. Regrettably, significant shrinkage accompanies this reduction in density and drying and is responsible for a significant loss in thermal insulating performance of the material.
An extensive study of this subject can be found in Bowles and Shirliffe: Development of a Canadian Standard for Urea Formaldehyde Thermal Wall Insulation, Thermal Insulation Performances, ASTM STP 718 and D. L. McElroy and R. P. Tye, Editors, American Society for Testing and Materials, 1980, pp 361-394. The description of these publications can be found on page 2 of the French text.
These studies have concluded that the shrinkage can be up to about 21% by volume and it can be estimated that such shrinkage reduces the thermal performance of the material by 60% with respect to the same material hypothetically filling the original space without shrinkage.
These undesirable results have been confirmed by numerous experiences carried out utilizing urea-formaldehyde foams.
One might expect the polyurethane foams to effectively replace the urea-formaldehyde foams because of their excellent thermal properties, their desirable mechanical properties, and their good dimensional stability.
In practice, however, it is found that this is not the case. For example, it is not possible or advantageous to simply inject such materials in the space between a wall and a partition because of the sharp increase in pressure which occurs when this material is injected into a wall. The expansion pressure of the foam appears to be of the order of kg/cm.sup.2 and the use of such materials require support for the walls of the cavities into which the polyurethane foam is introduced.
For example, if a metallic structure is filled in the factory with such foams, the metal panels frequently are distorted. Sandwich structures likewise formed with such foams are also distorted or deformed. If the polyurethane resin is injected between a supporting wall and a partition, the partition is frequently displaced, cracked or otherwise ruptured in a wholly unacceptable manner.
Of course, one can utilize specially reinforced partitions and other structures adapted to withstand the expansion pressures of polyurethane but, of course, this can only be done with loss of economic advantages of the polyurethane foam.
Furthermore, the polyurethane foams cannot be utilized effectively in existing structures without such reinforcements.
Polyurethane foams suitable for application by projection onto a surface have also been developed, for example, for insulating terrace roofings, albeit with the disadvantages discussed above. Furthermore, the moisture in concrete or support structures against which the polyurethane foam may be brought, tends to affect the hardening or setting of the resin. In the region where the foam contacts the support, the properties of the foam differ from those elsewhere.
Another difficulty frequently encountered in the use of polyurethane foams is the limited temperature at which it can be applied, namely, between 10.degree. and 30.degree. C. Thus in severe temperature extremes as may occur in the summer and in the winter, the use of polyurethane foams is drastically limited.
Yet another disadvantage of using polyurethane foams is the flammability characteristics thereof, a disadvantage which is also found with urea-formaldehyde foams.
The conventional polyurethane foams are somewhat flammable and cannot be utilized where there is a danger of ignition. This is especially the case in buildings and domiciles and for the insulation of roof, ceiling and like structures when sandwich panels are provided.
Naturally, various remedies to this problem have been developed and, for example, some firms have fabricated polyols for use in the production of polyurethane foams which enable them to be fire retardant and, for example, to conform to the M 1 class utilizing the epiradiation test described in French industrial standard NF P 92-501.
However, such foams contain nitrogen, chlorine or bromine or combinations thereof and, upon combination, emit opaque smoke or highly toxic vapors and thus, although they are less flammable, upon contact with a source of combustion, can give rise to other health disadvantages.
Epoxy resins have been found to be interesting in the production of thermally insulating foams, although their cost is so great that they have been utilized only effectively in industrial apparatus such as steam boilers of the like, which justify the higher cost. Furthermore, their combustion characteristics have limited their applicability in the same manner as polyurethane foams. Phenol-aldehyde resins, generally referred to as phenolic resins, have the advantage that they are exceptionally resistant to combustion for materials which are rich in carbon and hydrogen. Indeed, it is possible to form from them foams, which are non-flammable and are in the M 1 class for the epiradiation test as previously described.
It is for this reason that considerable effort has been expended in the development of such foams where there is a chance of combustion, for example, in building structures and in transport vehicles.
The usual method of producing a foam of a phenolic resin comprises mixing a resin known as a resol with an acidic compound in the presence of a surface active agent and a blowing agent, permitting the mixture to react under the exothermicity of the reaction until a fluid foam is formed which hardens progressively until the reaction concludes.
The resol based phenol-aldehyde resins which can be foamed in the manner described can be applied by the methods utilized with other resins, e.g. by casting, by projection onto a surface and by injection into a cavity. However, because of a variety of difficulties generally associated with the formation of a foam of such resins, the use thereof has been marginal to date.
As a matter of fact, considerable research has gone into the efforts to control the reaction and into the production of articles utilizing such resins without significant success.
For example, resol based phenol-aldehyde resins have poor reactivity. When a foamed resin is to be produced from two components, it is necessary to provide sufficient time for the two components to react in the mold or in the cavity to be filled or in the device for metering the two components into the mold or cavity. However, this time cannot be excessive because there is always the risk that the mixture will drain from the cavity, especially if the walls thereof are porous, as is the case in building structures. For urea-formaldehyde resins, the time required for the reaction is less than one minute which has been considered to be a suitable time period.
Within this time period, the foaming reactions should be rapid and hardening should follow directly with a sufficient exothermicity to completion and to eliminate volative materials.
Thus the desirable kinetic curve should increase exponentially with temperature after a period of about one minute. With resol resins, while it is possible to modify the reactivity by various means, e.g. controlling the concentration of the methylol group, the nature and concentration of the catalyst and hardener, or other aspects of the reaction, it is found that when the duration of contact is sufficient for the initial aspect of the reaction, the exothermicity of the reaction is insufficient to provide, at ambient temperature within an appropriate time of the order, say, of two minutes, a complete drying and hardening of the foam.
A second disadvantage of resol based resins used to date is that the viscosity of the resol resin for foaming is generally between 1,000 and 5,000 centipoises. It is possible to reduce this viscosity to slightly below 1,000 centipoises by increasing the concentration of water in the product but this reduces the reactivity of the resin and diminishes the properties of the final product as well as its dimensional stability. The viscosities of the resol resins which must be used do not permit the generation of light foams of the type which have been found to be satisfactory with urea-formaldehyde resins and indeed it is generally not possible to obtain a final density of the material significantly below 30 kg/m.sup.3 even under the best conditions.
Still another disadvantage of resol-based resins for the purposes described is the need to utilize large quantities of hardening agent or catalyst. Because of the poor reactivity and the higher apparent density of the foam, it is generally necessary to accelerate the reaction by utilizing comparably large quantities of hardening agents or catalysts. The compounds are generally acids, for example sulfonic acids, and when these are used in excess, they tend to increase the hydrophilic character of the foam and make the same corrosive.
Yet another disadvantage of the use of resol based resins for foam insulation is the limited temperature at which the foam can be generated, generally a temperature in the range of 10.degree. to 25.degree. C. Below 10.degree. C., the product is difficult to handle and there is a risk that it will solidify or plug the pumps, piping, nozzles and the mixing head. Furthermore, the reactivity is substantially reduced at temperatures below 10.degree. C.
At temperatures about 25.degree. C., the control fo the reaction can be lost. Consequently, one is generally limited in the time of application of the product to a narrower interval than that which has been found to be disadvantageous for the use of urea-formaldehyde resins.
It has also been found that the resol resins of conventional types have poor storage stability of shelf life. The resins tend to condense autogeneously with increase of their viscosity. The reaction occurs more readily as the storage temperature increases. Resol resins are indicated to have shelflives less than several weeks. Consequently it is difficult to stock the materials at various locations or to ship them long distances.
Finally, it should be mentioned that the conventional resol resins produce foamed products which have poor dimensional stability and which are somewhat brittle.
It has already been recognized that the mechanical strength properties of foamed resins formed from such resols are poor and especially that they have extremely low tensile strength.
Indeed, when dimensional stability is tested according to the French standard NFT 56122, i.e. after several cycles including a conditioning phase at ambient temperature and in a relatively constant humidity atmosphere, followed by a period in an oven, it is noted that there is a substantial dimensional variance. In suitable foams, this variation is minimal. A minimum variation under this test is necessary if degredation of the foam with time is to be avoided since the test represents an accelerated weathering or aging test. In fact, dimensional variations under such tests should be less than 2% and this value is difficult to guarantee when the foam is made of a resol resin of the type described.